It is known that polyurethane rigid foams can be blown with low-boiling alkanes. Cyclic alkanes are used to advantage here because they make an outstanding contribution to the thermal conductivity of the expanded material due to their low gaseous thermal conductivity. Cyclopentane is preferably used.
The beneficial properties when used as an insulator in domestic refrigerators have to be compared with a disadvantageous commercial situation. Thus, a specific quality of polystyrene inner container has to be used, as a result of the solvent properties of cyclopentane.
Furthermore, cyclopentane has the disadvantage, due to its relatively high boiling point of 49.degree. C., that it condenses at low temperatures such as are conventional during the use of polyurethane rigid foams as insulators in domestic refrigerators. Due to the undesired condensation of the blowing agent, a reduced pressure is produced in the cells which again has to be offset by an elevated foam strength or increased density.
Compared with the acyclic homologous pentane compounds, n-pentane and i-pentane, cyclopentane incurs higher manufacturing costs. n-pentane or i-pentane blown systems have been known for some time in the field of polyurethane rigid foams. However, the higher gaseous thermal conductivities, as compared with cyclopentane,which result in poorer thermal insulation capacity of the corresponding expanded systems is a disadvantage.
The object of the present invention was to develop a n-pentane or i-pentane blown rigid foam in which the disadvantages mentioned above are overcome and in particular in which low thermal conductivities are produced.
Surprisingly, it has now been found that polyol formulations based on specific polyethers and/or polyesters and polyisocyanates, which have a specific surface tension with respect to n-pentane or i-pentane as blowing agent, produce expanded materials with particularly low thermal conductivities.